OFDMA and SC-OFDMA
In a wireless communication network, such as the 3rd generation (3G) wireless cellular communication standard and the 3GPP long term evolution (LTE) standard, it is desired to concurrently support multiple services and multiple data rates for multiple users in a fixed bandwidth channel. One scheme adaptively modulates and codes symbols before transmission based on current channel estimates. Another option available in LTE, which uses orthogonal frequency division multiplexed access (OFDMA), is to exploit multi-user frequency diversity by assigning different sub-carriers or groups of sub-carriers to different users or UEs (user equipment, mobile station or transceiver). In the single carrier frequency division multiple access (SC-FDMA) uplink of LTE, in each user, the symbols are first together spread by means of a Discrete Fourier Transform (DFT) matrix and are then assigned to different sub-carriers. The network bandwidth can vary, for example, from 1.25 MHz to 20 MHz. The network bandwidth is partitioned into a number of subcarriers, e.g., 1024 subcarriers for a 10 MHz bandwidth.
The following standardization documents are applicable 36.211, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Channels and Modulation (Release 8), v 1.0.0 (2007-03); R1-01057, “Adaptive antenna switching for radio resource allocation in the EUTRA uplink,” Mitsubishi Electric/Nortel/NTT DoCoMo, 3GPP RAN1#48, St. Louis, USA; R1-071119, “A new DM-RS transmission scheme for antenna selection in E-UTRA uplink,” LGE, 3GPP RAN1#48, St. Louis, USA; and “Comparison of closed-loop antenna selection with open-loop transmit diversity (antenna switching within a transmit time interval (TTI)),” Mitsubishi Electric, 3GPP RAN1#47bis, Sorrento, Italy. According to the 3GPP standard, the base station (BS) is enhanced, and is called the “Evolved NodeB” (eNodeB). We use the terms BS and eNodeB interchangeably.
Multiple Input Multiple Output (MIMO)
In order to further increase the capacity of a wireless communication network in fading channel environments, multiple-input-multiple-output (MIMO) antenna technology can be used to increase the capacity of the network without an increase in bandwidth. Because the channels for different antennas can be quite different, MIMO increases robustness to fading and also enables multiple data streams to be transmitted concurrently.
Moreover, processing the signals received in spatial multiplexing schemes or with space-time trellis codes requires transceivers where the complexity can increase exponentially as a function of the number of antenna.
Antenna Selection
Antennas are relatively simple and cheap, while RF chains are considerably more complex and expensive. Antenna selection reduces some of the complexity drawbacks associated with MIMO networks. Antenna selection reduces the hardware complexity of transmitters and receivers in the transceivers by using fewer RF chains than the number of antennas.
In antenna selection, a subset of the set of available antennas is adaptively selected by a switch, and only signals for the selected subset of antennas are connected to the available RF chains for signal processing, which can be either transmitting or receiving. As used herein, the selected subset, in all cases, means one or more of all the available antennas in the set of antennas.
Pilot Tones or Reference Signals
In order to select the optimal subset of antennas, channels corresponding to available subsets of antennas need to be estimated, even though only a selected optimal subset of the antennas is eventually used for transmission.
This can be achieved by transmitting antenna selection signals, e.g., pilot tones, also called reference signals, from different antennas or antenna subsets. The different antenna subsets can transmit either the same pilot tones or use different ones. Let Nt denote the number of transmit antennas, Nr the number of receive antennas, and let Rt=Nt/Lt and Rr=Nr/Lr be integers. Then, the available transmit (receive) antenna elements can be partitioned into Rt (Rr) disjoint subsets. The pilot repetition approach repeats, for Rt×Rr times, a training sequence that is suitable for an Lt×Lr MIMO network. During each repetition of the training sequence, the transmit RF chains are connected to different subsets of antennas. Thus, at the end of the Rt×Rr repetitions, the receiver has a complete estimate of all the channels from the various transmit antennas to the various receive antennas.
In case of transmit antenna selection in frequency division duplex (FDD) networks, in which the forward and reverse links (channels) are not identical, the transceiver feeds back the optimal set of the selected subset of antennas to the transmitter. In reciprocal time division duplex (TDD) networks, the transmitter can perform the selection independently.
For indoor local area network (LAN) applications with slowly varying channels, antenna selection can be performed using a media access (MAC) layer protocol, see IEEE 802.11n wireless LAN draft specification, I. P802.11n/D1.0, “Draft amendment to Wireless LAN media access control (MAC) and physical layer (PHY) specifications: Enhancements for higher throughput,” Tech. Rep., March 2006.
Instead of extending the physical (PHY) layer preamble to include the extra training fields (repetitions) for the additional antennas, antenna selection training is done at the MAC layer by issuing commands to the physical layer to transmit and receive packets by different antenna subsets. The training information, which is a single standard training sequence for an Lt×Lr MIMO network, is embedded in the MAC header field.
SC-FDMA Structure in LTE
The basic uplink transmission scheme is described in 3GPP TR 25.814, v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is a single-carrier transmission (SC-FDMA) with cyclic prefix (CP) to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver.
Broadband Sounding Reference Signals (SRS)
The broadband SRS helps the eNodeB to estimate the entire frequency domain response of the uplink channel from the user to the eNodeB. This helps frequency-domain scheduling, in which a subcarrier is assigned, in principle, to the user with the best uplink channel gain for that subcarrier. Therefore, the broadband SRS can occupy the entire network bandwidth, e.g., 5 MHz or 10 MHz, or a portion thereof as determined by the eNodeB. In the latter case, the broadband SRS is frequency hopped over multiple transmissions in order to cover the entire network bandwidth.